Information leakage becomes a serious issue amid the advance of the information society, and a technique of preventing information leakage is expected. For example, as for digital data, a technique of encrypting digital data has been developed to prevent the third party from reading the contents of information even if the information is leaked to the third party. This technique is already utilized as a promising measure for preventing information leakage.
On the other hand, a technique of preventing information leakage of printed matters printed on a paper medium etc. is under development. Thus, the development of a technique capable of preventing information leakage of printed matters as well as the digital data is an urgent expectation.
Specific examples of the print image that expects a measure for preventing information leakage include bills for purchased goods, personal identification numbers such as credit-card numbers or social security numbers, medical charts, school grade reports, and lists of clients. Embodiments of the present invention may be applicable as a technique for countering the information leakage by encrypting an important portion of these images, for example.
To give examples of the Related Art regarding a technique of encrypting and decrypting printed matters, there are a method of dividing a document image into a plurality of blocks and scrambling the blocks (for example, U.S. Pat. No. 4,459,611), a method of scrambling blocks with inversion or mirror-reverse of each block (for example, Japanese Laid-open Patent Publication No. H08-179689), a method of scrambling blocks with a rotation of each block (for example, Japanese Patent No. 3609097), and a method of adding a reference mark for positional detection to an encrypted image and, upon decryption, detecting boundaries between blocks by using the reference mark to process the blocks for at least one of recording, scaling, rotating, shifting, and defect repairing (for example, Published Japanese Translation of a PCT application No. 9-504660).